According to a recent demand of society, a mass-reduction is strongly demanded in transport machines such as automobiles. A lot of steel sheets are used in the transport machines such as automobiles, and a use of high-strength materials for exterior sheets (body) or skeleton members is proceeded so as to fulfill the demand for mass-reduction. Hot-rolled steel sheets are used for underbody components such as arms and wheel disks. With regard to these underbody components, there is a concern of an effect on ride quality due to a decrease in rigidity; and therefore, thinning through high strengthening has not been positively examined.
However, since the demand for the mass-reduction has further increased, this demand is also made without exception to the underbody components. For example, the upper limit of the tensile strength of the hot-rolled steel sheet that is used in the related art is 590 MPa class; however, a use of steel sheets of 780 MPa class begins to be examined. Under this circumstance, a fatigue property and a corrosion resistance are required for the steel sheet in addition to a formability that commensurates with the strength.
With regard to the corrosion resistance among these properties, a steel sheet having a sufficient sheet thickness is used to secure rigidity in the related art. Therefore, even when the sheet thickness is reduced due to corrosion, an effect on properties of the components is small, and the corrosion resistance of the steel sheet is not seen as a problem. However, as described above, the thinning of a component has been directed, and a corrosion allowance to allow the reduction in sheet thickness due to corrosion has been reduced. Here, the corrosion allowance is a thickness that is enlarged in design in consideration of the amount of metal reduction due to corrosion during usage. In addition, simplification of chemical conversion processing and coating is considered to reduce a manufacturing cost. Therefore, it is necessary to pay more attention to a property or state in a surface of a steel material as compared to the related art.
When a hot-rolled steel sheet is applied to the underbody component, the hot-rolled steel sheet is shipped after being acid-pickled and coated with oil. Thereafter, the hot-rolled steel sheet is processed into components, and then the processed steel sheet is subjected to a chemical conversion processing and a coating process in many cases. Among properties of the hot-rolled steel sheet which are required for these treatment processes, particularly, a chemical conversion processability is most affected by the property and the state in the surface of the steel sheet, and has a great effect on the corrosion resistance.
In addition, since stress is repeatedly applied to strength members such as the underbody components, a fatigue property is required for the hot-rolled steel sheet.
Furthermore, since a sheared end portion is processed in many cases, a stretch flangeability (stretch-flange formability), that is, a hole expandability is also required for the hot-rolled steel sheet in many cases.
In addition to these, isotropy in properties of the material (hot-rolled steel sheet) during processing is gradually treated as important. In the case where anisotropy in a press formability or the like is small, a degree of freedom of collecting a blank for forming becomes high; and therefore, an improvement in a yield rate may be expected.
Since a remaining portion of the steel sheet after the blank for forming is collected is treated as a waste, it is necessary to allocate the blank so as to reduce the generation of the waste as much as possible. However, in the case where the anisotropy is present in the formability of the steel sheet, when a direction (for example, a more largely stretched (elongated) direction) of a component, in which a forming condition is strict, is allocated to a direction in which the formability (for example, stretch property (elongation property)) is inferior, an occurrence ratio of defects during forming becomes high. Therefore, the allocation direction of the blank is restricted. As a result, a yield ratio (smallness in an amount of generated waste) deteriorates as compared to a case in which the restriction is not present. This situation is reflected in the reason why the steel sheet having isotropic properties is preferred.
Suppression of occurrence of surface deterioration during forming is one of the properties to be required, and a countermeasure thereof is also demanded.
The surface deterioration is one of defects that are observed in a portion of the component after being press-molded, and it is well known that this is due to a minute unevenness. As one of the well-known methods for suppressing the surface deterioration, it is effective to make lengths of crystal grains of the material in a surface layer not be excessively large in a rolling direction.
An acid pickling property of the hot-rolled steel sheet is also gradually treated as important. In an acid-pickled surface (property and state of a surface after the acid pickling) of the hot-rolled steel sheet, the same smoothness as a cold-rolled steel sheet has not been required in the related art. However, consumer needs and the like vary, and there occurs a tendency that it is strongly preferred to make the surface as smooth as possible.
The smoothness of the acid-pickled surface is improved by lowering a concentration of hydrochloric acid in a hydrochloric acid aqueous solution that is used in the acid pickling and a temperature thereof. However, productivity decreases under the condition thereof; and therefore, a hot-rolled steel sheet having an acid pickling property superior to a steel sheet that is obtained until now is desirable.
Many technologies have been proposed which improve a fatigue property and a stretch flangeability of the steel sheet, and the present inventors also have promoted a research to optimize chemical components and a microstructure of the steel sheet.
On the other hand, the chemical conversion processability of the steel sheet depends on a Si content of the steel sheet, and it is well-known that the more the Si content is, the more inferior the chemical conversion processability becomes.
However, in the case where the steel sheet is highly strengthened by making Si be solid-solubilized in ferrite phases, a deterioration amount of ductility is not remarkably large. Therefore, Si is an element that is preferred to be used as much as possible in the manufacturing of the high-strength steel sheet. In addition, particularly, in the case where a steel sheet having both of high ductility and high strength is manufactured by combining the ferrite phases and hard phases such as martensite phases, Si is an element effective to secure a predetermined fraction ratio of the ferrite phases.
As a method of responding to these contradicting demands, a technology in which a part of Si is substituted by Al is proposed (for example, Patent Document 1).
Patent Document 1 discloses a hot-rolled steel sheet having a high tensile strength which contains less than 1% of Si and 0.005 to 1.0% of Al, and a method of producing the same. However, the production method disclosed in Patent Document 1 includes a process of heating a rough bar (a rough rolled material). The production method premised on the heating of the rough rolled material is special. As a result, there is a problem in that only limited business operators can execute the production method.
In general, facilities used in the process of producing the hot-rolled steel sheet include a heating furnace, a roughing mill, a descaling device, a finishing mill, a cooling device, and a coiler. Each of the respective facilities is disposed at an optimal position. Therefore, even when the advantage of heating the rough rolled material is wanted to be obtained, there is no space to provide a new facility, or a lot of modification on the facilities is necessary. As a result, the heating of the rough rolled material is not generalized yet. In addition, there is no description with respect to the chemical conversion properties of the steel sheet that is obtained by the technology disclosed in Patent Document 1.
On the other hand, Patent Document 2 discloses a hot-rolled steel sheet that contains Si and Al and is superior in the chemical conversion processability, and a method of producing the same.
However, in Patent Document 2, the upper limit of an Al content is specified to 0.1%, and it is described that in the case where the Al content exceeds this upper limit, the corrosion resistance deteriorates although the reason is not clear.
As described above, a hot-rolled steel sheet that contains at least 0.3% or more of Al together with Si and that is superior in the chemical conversion processability, and a method of producing the same are not found.